The Syn-6 would not be overkill at all. I would also consider a Wet Sounds Class-D HT-4 (110W rms x 4 @ 4ohm) for the in-boats and an HT-2 (575w rms x 1 @ 4) for the sub. This would be a tad more to the in-boats and about the same for the sub. Or, the Kicker Class-D IX500.4 (75W rms x 4 @ 4 ohm) for the in-boats and the IX500.2 (500w rms x 1 @ 4 ohm) for the sub. Theres probably half a dozen other hi-efficiant amp options out there as well, these are just a couple that came to mind.